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Precalculus
Trigonometric Identities
Time required
45 minutes
ID: 9848
Activity Overview
Students will verify, prove, and explore trigonometric identities symbolically, numerically, and
graphically.
Topic: Trigonometric Identities
 Use the Pythagorean Theorem to prove the trigonometric identities sin2 + cos2 = 1 and
sec2 = 1 + tan2.
 Verify trigonometric identities by graphing.
Teacher Preparation and Notes
 Prior to beginning the activity, students should download the VERITRIG program to their
calculators. This investigation has students exploring, proving, and verifying
trigonometric identities.

Students should already be familiar with right triangle trigonometric properties, similar
triangles, and the Pythagorean Theorem since much of the activity is based on these
fundamental theorems.

Students should begin by clearing out any functions from the  screen and turning off all
Stat Plots.

To download the student worksheet and calculator program, go to
education.ti.com/exchange and enter “9848” in the keyword search box.
Associated Materials
 TrigIdentities_Student.doc
 VERITRIG.8xp (program)
Suggested Related Activities
To download any activity listed, go to education.ti.com/exchange and enter the number in the
quick search box.
 Proof of Identity (TI-84 Plus family) — 9846
 Sine. It’s the Law. (TI-84 Plus family) — 11851
©2011 Texas Instruments Incorporated
Teacher Page
Trigonometric Identities
TImath.com
Precalculus
To start, students will need an empty graph window. They
should clear out any functions from the  screen and turn
off all Stat Plots. Make sure they are in degree mode. To
do this, press  and select DEGREE.
Problem 1 – Proving cos2 + sin2 = 1
In this problem, students will prove two of the basic
trigonometric identities using the Pythagorean Theorem.
Press  to access the Program menu.
Then choose the VERITRIG program and press . Select
1: PROVE ID 1.
Students will see a circle with a radius of 1 with a right
triangle in the first quadrant.
Students will label the right triangle using the Text tool.
Press   and arrow down to select Text.
Now students can position the cursor near the middle of
the hypotenuse and enter in the radius, 1. Then press  
to lock the alpha key on and enter in x, y, and  in their
respective places in a right triangle.
If students make a mistake, they can press   and select
ClrDraw. Then they need to run the program again.
On the worksheet, students will prove: cos2 + sin2 = 1.
1. x2 + y2 = 1
2. cos =
x
y
and sin =
1
1
3. Substituting x = cos and y = sin into x2 + y2 = 1 gives cos2 + sin2 = 1.
When finished, students can press   to exit the drawing.
©2011 Texas Instruments Incorporated
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Trigonometric Identities
TImath.com
Precalculus
Problem 2 – Proving sec2 = 1 + tan2
Students need to run the VERITRIG program and select
2: PROVE ID 2.
Students will see a new version of the unit circle with the
original right triangle (from Problem 1) and then a similar
triangle drawn to incorporate the original triangle.
Again, they need to use the Text tool and label the
original triangle.
Students are told on the worksheet that the length of the
base of the large triangle is 1, the height is Y and the
hypotenuse is X. Students need to use the Text tool to
label the large similar triangle accordingly.
Using the ratio given on the worksheet, students will prove sec2 = 1 + tan2. You may need
to explain to students that in both equations, (small ) and (large ), reference where that
part of the ratio came from and is not actually part of the ratio.
1
X
=
cos 
1
1 = (cos)(X)
X=
1
= sec
cos 
Y
1
=
sin 
cos
(Y)(cos) = (sin)(1)
Y=
sin
= tan
cos 
Substituting Y= tan and X = sec into the Pythagorean Theorem based on the larger
triangle: 12 + Y2 = X2
→1+ (tan)2 = (sec)2
1 + tan2 = sec2
Problem 3 – Numerical verification
Now that students have proved the two identities, they will numerically verify the two identities.
Students need to run the VERITRIG program and select NUM EXPLORE.
Students will see the same circle, but with three triangles
in the first quadrant. When they press , they will see the
x-values for cosine and the y-values for sine for each
angle measurement.
Students should observe that the values for cosine and
sine repeat but change signs depending on what
quadrant the angle is in.
©2011 Texas Instruments Incorporated
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Trigonometric Identities
TImath.com
Precalculus
These values are stored in the lists of the calculator with
the angle measurements in L1, the cosine values in L2,
and the sine values in L3.
To numerically verify cos2 + sin2 = 1; students are to
enter L22 + L32 at the top of L4.
Students will discover that they need to enter “1/cos ” to
3
get sec. The value of sec is undefined at 0 and
,
2
and the calculator will not evaluate these values within a
list. The same thing will happen to tan , which is equal to
sin
.
cos 
However, students should arrive at the fact that they can
graph the identities to verify the identity as the screen
shots illustrate. They should set the calculator to the trig
window by pressing  and select ZTrig. (Note: Students
will need to use the ClrDraw command to erase the unit
circle and turn off Plot1.)
Problem 4 – Verifying trig identities using graphing.
The previous problem leads into utilizing graphing
techniques to verify various trig identities.
Students are to determine whether the identity
sin2x = 1 – cos2x is true by entering sin2x into Y1 and
1 – cos2x into Y2. To set the bubble animation for the
second graph, students need to press  to the left of Y2 =.
Students need to select ZTrig for a trig window.
If the bubble runs over the same graph as the first, the
equation is a trig identity.
Explain to students that they can check any equation
using this method.
©2011 Texas Instruments Incorporated
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Trigonometric Identities